Electrochromic device having a self-cleaning hydrophilic coating

ABSTRACT

An electrochromic device is disclosed having a self-cleaning, hydrophilic optical coating. The electrochromic device preferably forms an external rearview mirror for a vehicle The coating preferably includes alternating layers of a photocatalytic material having a high index of refraction and a hydrophilic material having a low refractive index More specifically, the coating includes a first layer having a high refractive index, a second layer having a low refractive index, a third layer of titanium dioxide, and a fourth layer of silicon dioxide provided as an outermost layer. The disclosed optical coating exhibits a reflectance at the front surface of the reflective element that is less than about 20 percent, and has sufficient hydrophilic properties such that water droplets on a front surface of the optical coating exhibit a contact angle of less than about 20°.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/765,896 filed on Jan. 19. 2001. Now U.S. Pat. No. 6,447,123 B2, whichis a continuation of U.S. application Ser. No. 09/435,266 filed on Nov.5. 1999. Now U.S. Pat. No. 6,193,378 B1, which claims the benefit ofU.S. Provisional Application Serial No. 60/141,080, filed Jun. 25, 1999,which are hereby incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to electrochromic devices, andmore specifically relates to electrochromic rearview mirrors of avehicle.

To enable water droplets and mist to be readily removed from the windowsof a vehicle, the windows are typically coated with a hydrophobicmaterial that causes the water droplets to bead up on the outer surfaceof the window. These water beads are then either swept away bywindshield wipers or are blown off the window as the vehicle moves

It is equally desirable to clear external rearview mirrors of water.However, if a hydrophobic coating is applied to the external mirrors,the water beads formed on their surfaces cannot be effectively blown offsince such mirrors are relatively shielded from direct airflow resultingfrom vehicle movement Thus, water droplets or beads that are allowed toform on the surface of the mirrors remain on the mirror until theyevaporate or grow in size until they fall from their own weight. Thesewater droplets act as small lenses and distort the image reflected tothe driver. Further, when the water droplets evaporate, water spots areleft on the mirror, which are nearly as distracting as the waterdroplets that left the spots. In fog or high humidity, mist forms on thesurfaces of the external mirrors. Such a mist call be so dense that iteffectively renders the mirrors virtually unusable

In an attempt to overcome the above-noted problems, mirror manufacturershave provided a hydrophilic coating on the outer surface of the externalmirrors. See U.S. Pat. No. 5,594,585. One such hydrophilic coatingincludes a single layer of silicon dioxide (SiO₂) The SiO₂ layer isrelatively porous. Water on the mirror is absorbed uniformly across thesurface of the mirror into the pores of the SiO₂ layer and subsequentlyevaporates leaving no water spots. One problem with such single layercoatings of SiO₂ is that oil, grease, and other contaminants can alsofill the pores of the SiO₂ layer Many such contaminants, particularlyhydrocarbons like oil and grease, do not readily evaporate and henceclog the pores of the SiO₂ layer. When the pores of the SiO₂ layerbecome clogged with car wax, oil, and grease, the mirror surface becomeshydrophobic and hence the water on the mirror tends to bead leading tothe problems noted above.

A solution to the above problem pertaining to hydrophilic layers is toform the coating of a relatively thick layer (e.g., about 1000-3000 Å ormore) of titanium dioxide (TiO₂). See European Patent ApplicationPublication No. EPO 816 466 A1. This coating exhibits photocatalyticproperties when exposed to ultraviolet (UV) radiation. Morespecifically, the coating absorbs UV photons and, in the presence ofwater, generates highly reactive hydroxyl radicals that tend to oxidizeorganic materials that have collected in its pores or on its surface.Consequently, hydrocarbons, such as oil and grease, that have collectedon the mirror are converted to carbon dioxide (CO₂) and hence areeventually removed from the mirror whenever UV radiation impinges uponthe mirror surface. This particular coating is thus a self-cleaninghydrophilic coating.

One measure of the hydrophilicity of a particular coating is to measurethe contact angle that the sides of a water drop form with the surfaceof the coating An acceptable level of hydrophilicity as present in amirror when the contact angle is less than about 30°, and morepreferably, the hydrophilicity is less than about 20°. The aboveself-cleaning hydrophilic coating exhibits contact angles that decreasewhen exposed to UV radiation as a result of the self-cleaning action andthe hydrophilic effect of the coating The hydrophilic effect of thiscoating, however, tends to reverse over time when the mirror is notexposed to UV radiation.

The above self-cleaning hydrophilic coating can be improved by providinga film of about 150 to 1000 Å of SiO₂ on top of the relatively thickTiO₂ layer. See U.S. Pat. No. 5,854,708 This seems to enhance theself-cleaning nature of the TiO₂ layer by reducing the dosage of UVradiation required and by maintaining the hydrophilic effect of themirror over a longer period of time after the mirror is no longerexposed to UV radiation.

While the above hydrophilic coatings work well on conventional rearviewmirrors having a chrome or silver layer on the rear surface of a glasssubstrate, they have not been considered for use on electrochromicmirrors for several reasons. A first reason is that many of theabove-noted hydrophilic coatings introduce colored double images andincrease the low-end reflectivity of the electrochromic mirror. Forexample, commercially available, outside electrochromic mirrors existthat have a low-end reflectivity of about 10 percent and a high-endreflectivity of about 50 to 65 percent. By providing a hydrophiliccoating including a material such as TiO₂, which has a high index ofrefraction, on a glass surface of the mirror, a significant amount ofthe incident light is reflected at the glass/TiO₂ layer interfaceregardless of the variable reflectivity level of the mirror. Thus, thelow-end reflectivity would be increased accordingly. Such a higherlow-end reflectivity obviously significantly reduces the range ofvariable reflectance the mirror exhibits and thus reduces theeffectiveness of the mirror in reducing annoying glare from theheadlights of rearward vehicles.

Another reason that the prior hydrophilic coatings have not beenconsidered for use on electrochromic elements is that they impartsignificant coloration problems. Coatings such as those having a 1000 Ålayer of TiO₂ covered with a 150 Å layer of SiO₂, exhibit a very purplehue When used in a conventional mirror having chrome or silver appliedto the rear surface of a glass element, such coloration is effectivelyreduced by the highly reflective chrome or silver layer, since the colorneutral reflections from the highly reflective layer overwhelm thecoloration of the lower reflectivity, hydrophilic coating layer.However, if used on an electrochromic element, such a hydrophiliccoating would impart a very objectionable coloration, which is madeworse by other components in the electrochromic element that can alsointroduce color.

Due to the problems associated with providing a hydrophilic coating madeof TiO₂ on an electrochromic mirror, manufacturers of such mirrors haveopted to not use such hydrophilic coatings. As a result, electrochromicmirrors suffer from the above-noted adverse consequences caused by waterdrops and mist.

SUMMARY OF THE INVENTION

Accordingly, it is an aspect of the present invention to solve the aboveproblems by providing a hydrophilic coating suitable for use on anelectrochromic device, particularly for an electrochromic mirror. Toachieve these and other aspects and advantages, a rearview mirroraccording to the present invention comprises an electrochromic mirrorelement having a reflectivity that may be varied in response to anapplied voltage so as to exhibit at least a high reflectance state andlow reflectance state, and a hydrophilic optical coating applied to afront surface of the electrochromic mirror element. The rearview mirrorpreferably exhibits a reflectance of less than 20 percent in said lowreflectance state, and also preferably exhibits a C* value less thanabout 20 in both said high and low reflectance states so as to exhibitsubstantial color neutrality.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings

FIG. 1 is a front perspective view of an external electrochromicrearview mirror assembly constructed in accordance with the presentinvention; and

FIG. 2 is a cross section of the external electrochromic rearview mirrorassembly shown in FIG. 1 along line II—II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an external electrochromic rearview mirror assembly 10constructed in accordance with the present invention. As shown, mirrorassembly 10 generally includes a housing 15 and a mirror 20 movablymounted in housing 15. Housing 15 may have any conventional structuresuitably adapted for mounting assembly 10 to the exterior of a vehicle.

FIG. 2 shows an exemplary construction of mirror 20. As broadlydescribed herein, mirror 20 includes a reflective element 100 having areflectivity that may be varied in response to an applied voltage and anoptical coating 130 applied to a front surface 112 a of reflectiveelement 100. Reflective element 100 preferably includes a first (orfront) element 112 and a second (or rear) element 114 sealably bonded inspaced-apart relation to define a chamber Front element 112 has a frontsurface 112 a and a rear surface 112 b, and rear element 114 has a frontsurface 114 a and a rear surface 114 b. For purposes of furtherreference, front surface 112 a of front element 112 shall be referred toas the first surface, rear surface 112 b of front element 112 shall bereferred to as the second surface, front surface 114 a of rear element114 shall be referred to as the third surface, and rear surface 114 b ofrear element 114 shall be referred to as the fourth surface ofreflective element 100 Preferably, both elements 112 and 114 aretransparent and are scalably bonded by means of a seal member 116

Reflective element 100 also includes a transparent first electrode 118carried on one of second surface 112 b and third surface 114 a, and asecond electrode 120 carried on one of second surface 112 b and thirdsurface 114 a. Second electrode 120 may be reflective or transflective,or a separate reflector 122 may be provided on fourth surface 114 b ofmirror 100 in which case electrode 120 would be transparent. Preferably,however, second electrode 120 is reflective or transflective and thelayer referenced by numeral 122 is an opaque layer or omitted entirelyReflective element 100 also includes an electrochromic medium 124contained in the chamber in electrical contact with first and secondelectrodes 118 and 120.

Electrocbromic medium 124 includes electrochromic anodic and cathodicmaterials that can be grouped into the following categories:

(i) Single layer—the electrochromic medium is a single layer of materialwhich may include small nonhomogeneous regions and includessolution-phase devices where a material is contained in solution in theionically conducting electrolyte and remains in solution in theelectrolyte when electrochemically oxidized or reduced. Solution-phaseelectroactive materials may be contained in the continuous solutionphase of a cross-linked polymer matrix in accordance with the teachingsof U.S. Pat. No. 5,928,572, entitled “LAYER AND DEVICES COMPRISING SAME”or International Patent Application No. PCT/US98/05570 entitled“ELECTROCHROMIC POLYMERIC SOLID FILMS, MANUFACTURING ELECTROCHROMICDEVICES USING SUCH SOLID FILMS, AND PROCESSES FOR MAKING SUCH SOLIDFILMS AND DEVICES.”

At least three electroactive materials, at least two of which areelectrochromic, can be combined to give a pre-selected color asdescribed in U.S. Pat. No. 6,020,987 entitled “ELECTROCHROMIC MEDIUMCAPABLE OF PRODUCING A PRE-SELECTED COLOR.”

The anodic and cathodic materials can be combined or linked by abridging unit as described in International Application No.PCT/WO97/EP498 entitled “ELECTROCHROMIC SYSTEM.” It is also possible tolink anodic materials or cathodic materials by similar methods. Theconcepts described in these applications can further be combined toyield a variety of electrochromic materials that are linked.

Additionally, a single layer medium includes the medium where the anodicand cathodic materials can be incorporated into the polymer matrix asdescribed in International Application No. PCT/WO98/EP3862 entitled“ELECTROCHROMIC POLYMER SYSTEM” or International Patent Application No.PCT/US98/05570 entitled “ELECTROCHROMIC POLYMERIC SOLID FILMS,MANUFACTURING ELECTROCHROMIC DEVICES USING SUCH SOLID FILMS, ANDPROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES.”

Also included is a medium where one or more materials in the mediumundergoes a change in phase during the operation of the device, orexample, a deposition system where a material contained in solution inthe ionically conducting electrolyte, which forms a layer or partiallayer on the electronically conducting electrode when electrochemicallyoxidized or reduced.

(ii) Multilayer—the medium is made up in layers and includes at leastone material attached directly to an electronically conducting electrodeor confined in close proximity thereto, which remains attached orconfined when electrochemically oxidized or reduced. Examples of thistype of electrochromic medium are the metal oxide films, such astungsten oxide, iridium oxide, nickel oxide, and vanadium oxide. Amedium, which contains one or more organic electrochromic layers, suchas polythiophene, polyaniline, or polypyrrole attached to the electrode,would also be considered a multilayer medium.

In addition, the electrochromic medium may also contain other materials,such as light absorbers, light stabilizers, thermal stabilizers,antioxidants, thickeners, or viscosity modifiers

Because reflective element 100 may have essentially any structure, thedetails of such structures are not further described. Examples ofpreferred electrochromic mirror constructions are disclosed in U.S. Pat.No. 4,902,108, entitled “SINGLE-COMPARTMENT, SELF-ERASING,SOLUTION-PHASE ELECTROCHROMIC DEVICES SOLUTIONS FOR USE THEREIN, ANDUSES THEREOF,” issued Feb. 20, 1990, to H. J. Byker; Canadian Patent No.1,300,945, entitled “AUTOMATIC REARVIEW MIRROR SYSTEM FOR AUTOMOTIVEVEHICLES,” issued May 19, 1992, to J. H. Bechtel et al.; U.S. Pat. No.5,128,799, entitled “VARIABLE REFLECTANCE MOTOR VEHICLE MIRROR,” issuedJul. 7, 1992, to H. J. Byker; U.S. Pat. No. 5,202,787, entitled“ELECTRO-OPTIC DEVICE,” issued Apr. 13, 1993 to H. J. Byker et al. U.S.Pat. No. 5,204,778, entitled “CONTROL SYSTEM FOR AUTOMATIC REARVIEWMIRRORS,” issued Apr. 20, 1993, to J. H. Bechtel, U.S. Pat. No.5,278,693, entitled “TINTED SOLUTION-PHASE ELECTROCHROMIC MIRRORS,”issued Jan. 11, 1994, to D. A. Theiste et al.; U.S. Pat. No. 5,280,380,entitled “UV STABILIZED COMPOSITIONS AND METHODS,” issued Jan. 18, 1994,to H. J. Byker; U.S. Pat. No. 5,282,077, entitled “VARIABLE REFLECTANCEMIRROR,” issued Jan. 25, 1994, to H. J. Byker, U.S. Pat. No. 5,294,376,entitled “BIPYRIDINIUM SALT SOLUTIONS,” issued Mar. 15, 1994, to H. J.Byker; U.S. Pat. No. 5,336,448, entitled “ELECTROCHROMIC DEVICES WITHBIPYRIDINIUM SALT SOLUTIONS,” issued Aug. 9, 1994, to H. J. Byker; U.S.Pat. No. 5,434,407, entitled “AUTOMATIC REARVIEW MIRROR INCORPORATINGLIGHT PIPE,” issued Jan. 18, 1995, to F. T. Bauer et al.; U.S. Pat. No.5,448,397, entitled “OUTSIDE AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVEVEHICLES,” issued Sep. 5, 1995, to W. L. Tonar; U.S. Pat. No. 5,451,822,entitled “ELECTRONIC CONTROL SYSTEM,” issued Sep. 19, 1995, to J. H.Bechtel et al.; U.S. Pat. No. 5,818,625, entitled “ELECTROCHROMICREARVIEW MIRROR INCORPORATING A THIRD SURFACE METAL REFLECTOR,” byJeffrey A. Forgette et al.; and U.S. patent application Ser. No.09/158,423, entitled “IMPROVED SEAL FOR ELECTROCHROMIC DEVICES,” filedon Sep. 21, 1998. Each of these patents and the patent application arecommonly assigned with the present invention and the disclosures ofeach, including the references contained therein, are herebyincorporated herein in their entirety by reference.

If the mirror assembly includes a signal light, display, or otherindicia behind the reflective electrode or reflective layer ofreflective element 100, reflective element 100 is preferably constructedas disclosed in commonly assigned U.S. patent application Ser. No.09/311,955, entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING ATHIRD SURFACE METAL REFLECTOR AND A DISPLAY/SIGNAL LIGHT,” filed on May14, 1999, by W. L. Tonar et al., the disclosure of which is incorporatedherein by reference. If reflective element 100 is convex or aspheric, asis common for passenger-side external rearview mirrors as well asexternal driver-side rearview mirrors of cars in Japan and Europe,reflective element 100 may be made using thinner elements 112 and 114while using a polymer matrix in the chamber formed therebetween as isdisclosed in commonly assigned U.S. Pat. No. 5,940,201 entitled“ELECTROCHROMIC MIRROR WITH TWO THIN GLASS ELEMENTS AND A GELLEDELECTROCHROMIC MEDIUM,” filed on Apr. 2, 1997. The entire disclosure,including the references contained therein, of this U.S. Patent isincorporated herein by reference. The addition of the combinedreflector/electrode 120 onto third surface 114 a of reflective element100 further helps remove any residual double imaging resulting from thetwo glass elements being out of parallel. The electrochromic element ofthe present invention is preferably color neutral. In a color neutralelectrochromic element, the element darkens to a gray color, which ismore ascetically pleasing than any other color when used in anelectrochromic mirror. U.S. Pat. No. 6,020,987, entitled “ELECTROCHROMICMEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR” discloseselectrochromic media that are perceived to be gray throughout theirnormal range of operation. The entire disclosure of this patent ishereby incorporated herein by reference. U.S. patent application Ser.No. 09/311,955 entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING ATHIRD SURFACE METAL REFLECTOR AND A DISPLAY/SIGNAL LIGHT” disclosesadditional electrochromic mirrors that exhibit substantial colorneutrality while enabling displays to be positioned behind thereflective surface of the electrochromic mirror. The entire disclosureof this application is hereby incorporated herein by reference.

In addition to reflective element 100, mirror 20 further includes anoptical coating 130 Optical coating 130 is a self-cleaning hydrophilicoptical coating. Optical coating 130 preferably exhibits a reflectanceat first surface 112 a of reflective element 100 that is less than about20 percent. If the reflectance at first surface 112 a is greater thanabout 20 percent, noticeable double-imaging results, and the range ofvariable reflectance of reflective element 100 is significantly reduced.The electrochromic mirror as a unit should have a reflectance of lessthan about 20 percent in its lowest reflectance state, and morepreferably should have a reflectance of less than about 10 percent.

Optical coating 130 also is preferably sufficiently hydrophilic suchthat water droplets on a front surface of coating 130 exhibit a contactangle of less than about 30°, more preferably less than about 20°, andmost preferably less than about 10°. If the contact angle is greaterthan about 30°, the coating 130 exhibits insufficient hydrophilicproperties to prevent distracting water beads from forming Opticalcoating 130 should also exhibit self cleaning properties whereby thehydrophilic properties may be restored following exposure to UVradiation.

In one embodiment, optical coating 130 includes at least four layers ofalternating high and low refractive index. Specifically, as shown inFIG. 2, optical coating 130 includes, in sequence, a first layer 132having a high refractive index, a second layer 134 having a lowrefractive index, a third layer 136 having a high refractive index, anda fourth layer 138 having a low refractive index. Preferably, thirdlayer 136 is made of a photocatalytic material, and fourth layer 138 ismade of a material that will enhance the hydrophilic properties of thephotocatalytic layer 136 by generating hydroxyl groups on its surfaceSuitable hydrophilic enhancement materials include SiO₂ and Al₂O₃, withSiO₂ being most preferred. Suitable photocatalytic materials includeTiO₂, ZnO, ZnO₂, SnO₂, ZnS, CdS, CdSe, Nb₂O₅, KTaNbO₃, KTaO₃, SrTiO₃,WO₃, Bi2O₃, Fe2O₃, and GaP, with TiO₂ being most preferred By making theoutermost layers TiO₂ and SiO₂, coating 130 exhibits good self-cleaninghydrophilic properties similar to those obtained by the prior arthydrophilic coatings applied to conventional mirrors having a reflectorprovided on the rear surface of a single front glass element.Preferably, the thickness of the SiO outer layer is less than about 800Å. If the SiO₂ outer layer is too thick (e.g., more than about 1000 Å),the underlying photocatalytic layer will not be able to “clean” the SiO₂hydrophilic outer layer, at least not within a short time period. Thetwo additional layers (layers 132 and 134) are provided to reduce theundesirable reflectance levels at the front surface of reflectiveelement 100. Preferably, layer 132 is made of a photocatalytic materialand second layer 134 is made of a hydrophilic enhancement material so asto contribute to the hydrophilic and photocatalytic properties of thecoating. Thus, layer 132 may be made of any one of the photocatalyticmaterials described above or mixtures thereof, and layer 134 may be madeof any of the hydrophilic enhancement materials described above ormixtures thereof. Preferably layer 132 is made of TiO₂, and layer 134 ismade of SiO₂.

An alternative technique to using a high index layer and low index layerbetween the glass and the layer that is primarily comprised ofphotocatalytic metal oxide (i e, layer 136) to obtain all of the desiredproperties while maintaining a minimum top layer thickness of primarilysilica is to use a layer, or layers, of intermediate index. Thislayer(s) could be a single material such as tin oxide or a mixture ofmaterials such as a blend of titania and silica. Among the materialsthat have been modeled as potentially useful are blends of titania andsilica, which can be obtained through sol-gel deposition as well asother means, and tin oxide One can use a graded index between the glassand layer primarily composed of photocatalytic material as well.

Additionally, one can obtain roughly the same color and reflectanceproperties with a thinner top layer or possibly no top layer containingprimarily silica if the index of the photocatalytic layer is loweredsomewhat by blending materials, as would be the case, for example, for atitania and silica mixture deposited by sol-gel. The lower index of thetitania and silica blend layer imparts less reflectivity, requires lesscompensation optically, and therefore allows for a thinner top layer.This thinner top layer should allow for more of the photocatalyticeffect to reach surface contaminants.

The index of refraction of a titania film obtained from a given coatingsystem can vary substantially with the choice of coating conditions andcould be chosen to give the lowest index possible while maintainingsufficient amounts of anatase or rutile form in the film anddemonstrating adequate abrasion resistance and physical durability. Thelower index obtained in this fashion would yield similar advantages tolowering the index by mixing titania with a lower index material. RonWilley, in his book “Practical Design and Production of Optical ThinFilms,” Marcel Dekker, 1996, cites an experiment where temperature ofthe substrate, partial pressure of oxygen, and speed of deposition varythe index of refraction of the titania deposited from about n=2.1 ton=2.4

Materials used for transparent second surface conductors are typicallymaterials whose index of refraction is about 1.9 or greater and havethen color minimized by using half wave thickness multiples or by usingthe thinnest layer possible for the application or by the use of one ofseveral “non-iridescent glass structures.” These non-iridescentstructures will typically use either a high and low index layer underthe high index conductive coating (see, for example, U.S. Pat. Nos.4,377,613 and 4,419,386 by Roy Gordon), or an intermediate index layer(see U.S. Pat. No. 4,308,316 by Roy Gordon) or graded index layer (seeU.S. Pat. No. 4,440,822 by Roy Gordon).

Fluorine doped tin oxide conductors using a non-iridescent structure arecommercially available from Libbey-Owens-Ford and are used as the secondsurface transparent conductors in most inside automotive electrochromicmirrors produced at the present time. The dark state color of devicesusing this second surface coating stack is superior to that of elementsusing optical half wave thickness indium tin oxide (ITO) when it is usedas a second surface conductive coating. Drawbacks of this non-iridescentcoating are mentioned elsewhere in this document. Hydrophilic andphotocatalytic coating stacks with less than about 800 Å silica toplayer, such as 1000 Å titania 500 Å silica, would still impartunacceptable color and/or reflectivity when used as a first surfacecoating stack in conjunction with this non-iridescent second surfaceconductor and other non-iridescent second surface structures, per theprevious paragraph, that are not designed to compensate for the color ofhydrophilic coating stacks on the opposing surface. Techniques wouldstill need to be applied per the present embodiment at the first surfaceto reduce C* of the system in the dark state if these coatings were usedon the second surface.

ITO layers typically used are either very thin (approximately 200-250 Å)which minimizes the optical effect of the material by making it as thinas possible while maintaining sheet resistances adequate for manydisplay devices, or multiples of half wave optical thickness (about 1400Å), which minimizes the overall reflectivity of the coating. In eithercase, the addition of photocatalytic hydrophilic coating stacks onopposing surfaces per the previous paragraph would create unacceptablecolor and/or reflectivity in conjunction with the use of these layerthicknesses of ITO used as the second surface conductor. Again,techniques would need to be applied per the present embodiment at thefirst surface to reduce the C* of the system in the dark state.

In somewhat analogous fashion, for modification of the firstsurface-coating stack to optimize the color and reflectivity of thesystem containing both first and second surface coatings, one can modifythe second surface-coating stack to optimize the color of the system.One would do this by essentially creating a compensating color at thesecond surface in order to make reflectance of the system more uniformacross the visible spectrum, while still maintaining relatively lowoverall reflectance For example, the 1000 Å titania 500 Å silica stackdiscussed in several places within this document has a reddish-purplecolor due to having somewhat higher reflectance in both the violet andred portions of the spectrum than it has in the green. A second surfacecoating with green color, such as ¾ wave optical thickness ITO, willresult in a lower C* value for the dark state system than a system witha more standard thickness of ITO of half wave optical thickness, whichis not green in color. Additionally, one can modify thicknesses oflayers or choose materials with somewhat different indices in thenon-iridescent structures mentioned in order to create a compensatingcolor second surface as well.

Another method of color compensating the first surface is throughpre-selecting the color of the electrochromic medium in the dark statein accordance with the teachings of commonly assigned U.S. Pat. No.6,020,987, entitled “ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING APRE-SELECTED COLOR.” Again, by using first surface coatings of 1000 Åtitania followed by 500 Å silica as an example, the followingmodification would assist in lowering the C* value of an electrochromicmirror when activated. If, in that case, the color of theelectrocliromic medium was selected so that it was less absorbing in thegreen region when activated, the higher reflection of green wavelengthsof light from the third or fourth surface reflector of the element wouldhelp balance the reflection of the unit in the dark state.

Combinations of the aforementioned concepts for the first, secondsurface, and electrochromic medium are also potentially advantageous forthe design

It is known that the optical properties for a deposited film varydepending on deposition conditions that include partial pressure ofoxygen gas, temperature of the substrate speed of deposition, and thelike. In particular, the index of refraction for a particular set ofparameters on a particular system will affect the optimum layerthicknesses for obtaining the optical properties being discussed.

The discussions regarding the photocatalytic and hydrophilic propertiesof titania and like photocatalytic materials and silica and likehydrophilic materials are generally applicable to layers of mixedmaterials as long as the mixtures retain the basic properties ofphotocatalytic activity and/or hydrophilicity. Abrasion resistance isalso a major consideration in the outermost layer. UP 0816466A1describes an abrasion resistant, photocatalytic, hydrophilic layer ofsilica blended titania, as well as a layer of tin oxide blended titaniawith similar properties. U.S. Pat. No. 5,755,867 describesphotocatalytic blends of silica and titania obtained through use ofthese mixtures. These coatings would likely require modifications tochange their optical properties suitable for use on an electrochromicdevice The potential advantages of these optical property modificationsto this invention are discussed further below.

In some variations of this invention, it may be preferable to include alayer of material between the substrate, especially if it is soda limeglass, and the photocatalytic layer(s) to serve as a barrier againstsodium leaching in particular If this layer is close to the index ofrefraction of the substrate, such as silica on soda lime glass, it willnot affect the optical properties of the system greatly and should notbe considered as circumventing the spirit of the invention with regardsto contrasting optical properties between layers.

To expedite the evaporation of water on the mirror and prevent thefreezing of thin films of water on the mirror, a heating element 122 mayoptionally be provided on the fourth surface 114 b of reflective element100. Alternatively, one of the transparent front surface films could beformed of an electrically conductive material and hence function as aheater.

To illustrate the properties and advantages of the present invention, anexample is provided below. The following illustrative example is notintended to limit the scope of the present invention but to illustrateits application and use. In this example, references are made to thespectral properties of an electrochromic mirror constructed inaccordance with the parameters specified in the example. In discussingcolors, it is useful to refer to the Commission Internationale deI'Eclairage's (CIE) 1976 CIELAB Chromaticity Diagram (commonly referredto as the L*a*b* chart). The technology of color is relatively complex,but a fairly comprehensive discussion is given by F. W. Billmeyer and M.Saltzman in Principles of Color Technology, 2nd Edition, J. Wiley andSons Inc. (1981), and the present disclosure, as it relates to colortechnology and terminology, generally follows that discussion. On theL*a*b* chart, L* defines lightness, a* denotes the red/green value, andb* denotes the yellow/blue value. Each of the electrochromic media hasan absorption spectra at each particular voltage that may be convertedto a three-number designation, their L*a*b* values. To calculate a setof color coordinates, such as L*a*b* values, from the spectraltransmission or reflectance, two additional items are required. One isthe spectral power distribution of the source or illuminant. The presentdisclosure uses CIE Standard Illuminant D₆₅. The second item needed isthe spectral response of the observer. The present disclosure uses the2-degree CIE standard observer. The illuminant/observer combination usedis represented as D₆₅/2 degree. Many of the examples below refer to avalue Y from the 1931 CIE Standard since it corresponds more closely tothe reflectance than L*. The value C*, which is also described below, isequal to the square root of (a*)²+(b*)², and hence, provides a measurefor quantifying color neutrality. To obtain an electrochromic mirrorhaving relative color neutrality, the C* value of the mirror should beless than 20. Preferably, the C* value is less than 15, and morepreferably is less than about 10.

EXAMPLE

Two identical electrochromic mirrors were constructed having a rearelement made with 2.2 mm thick glass with a layer of chrome applied tothe front surface of the rear element and a layer of rhodium applied ontop of the layer of chrome using vacuum deposition. Both mirrorsincluded a front transparent element made of 1.1 mm thick glass, whichwas coated on its rear surface with a transparent conductive ITO coatingof ½ wave optical thickness The front surfaces of the front transparentelements were covered by a coating that included a first layer of 200 Åthick TiO₂, a second layer of 250 Å thick SiO₂, a third layer of 1000 ÅTiO₂, and a fourth layer of 500 Å thick SiO₂. For each mirror, an epoxyseal was laid about the perimeter of the two coated glass substratesexcept for a small port used to vacuum fill the cell with electrochromicsolution. The seal had a thickness of about 137 microns maintained byglass spacer beads The elements were filled with an electrochromicsolution including propylene carbonate containing 3 percent by weightpolymethylmethacrylate, 30 mM Tinuvin P (UV absorber), 38 mMN,N′-dioctyl-4, 4′bipyridinium bis(tetrafluoroborate), 27 mM5,10-dihydrodimethylphenazine and the ports were then plugged with a UVcurable adhesive. Electrical contact buss clips were electricallycoupled to the transparent conductors.

In the high reflectance state (with no potential applied to the contactbuss clips), the electrochromic mirrors had the following averagedvalues: L*=78.26, a*=−2.96, b*=4.25, C*=5.18, and Y=53.7. In the lowestreflectance state (with a potential of 1.2 V applied), theelectrochromic mirrors had the following averaged values: L*=36.86,a*=6.59, b*=−3.51, C*=7.5, and Y=9.46. The average contact angle that adrop of water formed on the surfaces of the electrochromic mirrors afterit was cleaned was 7°.

For purposes of comparison, two similar electrochromic mirrors wereconstructed, but without any first surface coating. These two mirrorshad identical construction. In the high reflectance state, theelectrochromic mirrors had the following averaged values: L*78.93,a*=−2.37, b*=2.55, C*=3.48, and Y=54.8 1. In the lowest reflectancestate, the electrochromic mirrors had the following averaged values:L*=29.46, a*=0.55, b*=−16.28, C*=16.29, and Y=6.02. As this comparisonshows, the electrochromic mirrors having the inventive hydrophiliccoating unexpectedly and surprisingly had better color neutrality thansimilarly constructed electrochromic mirrors not having such ahydrophilic coating. Additionally, the comparison shows that theaddition of the hydrophilic coating does not appreciably increase thelow-end reflectance of the mirrors.

The present invention thus provides a hydrophilic coating that not onlyis suitable for an electrochromic device, but actually improves thecolor neutrality of the device

Although the example cited above uses a vacuum deposition technique toapply the coating, these coatings can also be applied by conventionalsol-gel techniques. In this approach, the glass is coated with a metalalkoxide made from precursors such as tetra isopropyl titanate, tetraethyl ortho silicate, or the like. These metal alkoxides can be blendedor mixed in various proportions and coated onto glass usually from analcohol solution after being partially hydrolyzed and condensed toincrease the molecular weight by forming metal oxygen metal bonds. Thesecoating solutions of metal alkoxides can be applied to glass substratesby a number of means such as dip coating, spin coating, or spraycoating. These coatings are then fired to convert the metal alkoxide toa metal oxide typically at temperatures above 450° C. Very uniform anddurable thin film can be formed using this method. Since a vacuumprocess is not involved, these films are relatively inexpensive toproduce. Multiple films with different compositions can be built upprior to firing by coating and drying between applications. Thisapproach can be very useful to produce inexpensive hydrophilic coatingson glass for mirrors, especially convex or aspheric mirrors that aremade from bent glass. In order to bend the glass, the glass must beheated to temperatures above 550° C. If the sol-gel coatings are appliedto the flat glass substrate before bending (typically on what will bethe convex surface of the finished mirror), the coatings will fire to adurable metal oxide during the bending process. Thus, a hydrophiliccoating can be applied to bent glass substrates for little additionalcost. Since the majority of outside mirrors used in the world today aremade from bent glass, this approach has major cost benefits It should benoted that some or all of the coatings could be applied by this sol-gelprocess with the remainder of the coating(s) applied by a vacuumprocess, such as sputtering or E-beam deposition For example, the firsthigh index layer and low index layer of, for instance, TiO₂ and SiO₂,could be applied by a sol-gel gel technique and then the top TiO₂ andSiO₂ layer applied by sputtering. This would simplify the requirementsof the coating equipment and yield cost savings. It is desirable toprevent migration of ions, such as sodium, from soda lime glasssubstrates into the photocatalytic layer. The sodium ion migration rateis temperature dependent and occurs more rapidly at high glass bendingtemperatures. A sol-gel formed silica or doped silica layer, forinstance phosphorous doped silica, is effective in reducing sodiummigration. This barrier underlayer can be applied using a sol-gelprocess. This silica layer could be applied first to the base glass orincorporated into the hydrophilic stack between the photocatalytic layerand the glass.

In general, the present invention is applicable to any electrochromicelement including architectural windows and skylights, automobilewindows, rearview mirrors, and sunroofs. With respect to rearviewmirrors, the present invention is primarily intended for outside mirrorsdue to the increased likelihood that they will become foggy or coveredwith mist. Inside and outside rearview mirrors may be slightly differentin configuration. For example, the shape of the front glass element ofan inside mirror is generally longer and narrower than outside mirrors.There are also some different performance standards placed on an insidemirror compared with outside mirrors For example, an inside mirrorgenerally, when fully cleared, should have a reflectance value of about70 percent to about 85 percent or higher, whereas the outside mirrorsoften have a reflectance of about 50 percent to about 65 percent Also,in the United States (as supplied by the automobile manufacturers), thepassenger-side mirror typically has a non-planar spherically bent orconvex shape, whereas the driver-side mirror 111 a and inside mirror 110presently must be flat In Europe, the driver-side mirror 111 a iscommonly flat or aspheric, whereas the passenger-side mirror 111 b has aconvex shape In Japan, both outside mirrors have a non-planar convexshape.

The fact that outside rearview mirrors are often non-planar raisesadditional limitations on their design. For example, the transparentconductive layer applied to the rear surface of a non-planar frontelement is typically not made of fluorine-doped tin oxide, which iscommonly used in planar mirrors, because the tin oxide coating cancomplicate the bending process and it is not commercially available onglass thinner than 2.3 mm. Thus, such bent mirrors typically utilize alayer of ITO as the front transparent conductor ITO, however, isslightly colored and adversely introduces blue coloration into thereflected image as viewed by the driver. The color introduced by an ITOlayer applied to the second surface of the element may be neutralized byutilizing an optical coating on the first surface of the electrochromicelement To illustrate this effect, a glass element coated with a halfwave thick ITO layer was constructed as was a glass element coated witha half wave thick ITO layer on one side and the hydrophilic coatingdescribed in the above example on the other side. The ITO-coated glasswithout the hydrophilic coating had the following properties: L*=37.09,a*=8.52, b*=−21.12, C*=22.82, and a first/second surface spectralreflectance of Y=9.58. By contrast, the ITO-coated glass that includedthe inventive hydrophilic coating of the above-described exampleexhibited the following properties L*=42 02, a*=2.34, b*=−8.12 C*=8.51,and a first/second surface spectral reflectance Y=12.51 As evidenced bythe significantly reduced C* value, the hydrophilic coating serves as acolor suppression coating by noticeably improving the coloration of aglass element coated with ITO Because outside rearview mirrors are oftenbent and include ITO as a transparent conductor, the ability to improvethe color of the front coated element by adding a color suppressioncoating to the opposite side of the bent glass provides manymanufacturing advantages.

Other light attenuating devices, such as scattered particle displays orliquid crystal displays, can also benefit from the application of theseprinciples. In devices where the light attenuating layer is between twopieces of glass or plastic, the same basic constraints and solutions tothose constraints will apply. The color and reflectivity of a firstsurface hydrophilic layer or layer stack can impart substantial colorand reflectivity to the device in the darkened state even when thisfirst surface layer stack does not appreciably affect the bright statecharacteristics. Adjustments to the first surface layer stack similar tothose discussed for an electrochromic device will, therefore, affect thecolor and/or reflectivity of the darkened device advantageously. Thesame will apply to adjustments made to the second surface of the deviceor to the color of the darkening layer itself.

These principles can also be applied to devices such as insulatedwindows where the light attenuating device may be contained within astructure having, for example, an additional outer and inner pane ofglass for insulation purposes. The hydrophilic coating would, in thiscase, need to be on the outside of the multilayer structure, but wouldaffect the color of the darkened device similarly. The techniquesdiscussed could be advantageously applied in this structure Thosefamiliar with the art can see that coatings could or would betransferred to a different surface or surfaces of the device because ofthe additional surfaces either available or interposing within thedevice

The above description is considered that of the preferred embodimentsonly Modifications of the invention will occur to those skilled in theart and to those who make or use the invention. Therefore, it isunderstood that the embodiments shown in the drawings and describedabove are merely for illustrative purposes and not intended to limit thescope of the invention, which is defined by the following claims asinterpreted according to the principles of patent law, including theDoctrine of Equivalents.

The invention claimed is:
 1. An electrochromic device, comprising: afirst substantially transparent element having a front surface and arear surface, wherein a self-cleaning, hydrophilic coating is applied toat least a portion of the front surface, and further wherein anelectrically conductive material is applied to at least a portion of therear surface; a second element having a front surface and a rearsurface, wherein an electrically conductive material is applied to atleast a portion of the front surface; an electrochromic medium containedwithin a chamber positioned between the first and second elements; andwherein the electrochromic device exhibits a C* value of less thanapproximately 20 in both a high reflectance state and a low reflectancestate.
 2. The electrochromic device according to claim 1, wherein theelectrochromic medium comprises at least one solvent, an anodicmaterial, and a cathodic material.
 3. The electrochromic deviceaccording to claim 1, wherein the self-cleaning, hydrophilic coating issufficiently hydrophilic such that water droplets on a front surface ofthe self-cleaning, hydrophilic coating exhibit a contact angle of lessthan about 30 degrees.
 4. The electrochromic device according to claim1, wherein the self-cleaning, hydrophilic coating is sufficientlyhydrophilic such that water droplets on a front surface of theself-cleaning, hydrophilic coating exhibit a contact angle of less thanabout 20 degrees.
 5. The electrochromic device according to claim 1,wherein the self-cleaning, hydrophilic coating is sufficientlyhydrophilic such that water droplets on a front surface of theself-cleaning, hydrophilic coating exhibit a contact angle of less thanabout 10 degrees.
 6. The electrochromic device according to claim 5,wherein the self-cleaning, hydrophilic coating includes at least onelayer of photocatalytic material having a thickness of less thanapproximately 300 angstroms.
 7. The electrochromic device according toclaim 5, wherein the self-cleaning, hydrophilic coating includes atleast one layer of photocatalytic material having a thickness of lessthan approximately 500 angstroms.
 8. An electrochromic device,comprising: a first substantially transparent element having a frontsurface and a rear surface, wherein a self-cleaning, hydrophilic coatingis applied to at least a portion of the front surface, and furtherwherein an electrically conductive material is applied to at least aportion of the rear surface; a second element having a front surface anda rear surface, wherein an electrically conductive material is appliedto at least a portion of the front surface; an electrochromic mediumcontained within a chamber positioned between the first and secondelements; and wherein the electrochromic device exhibits a C* value ofless than approximately 25 in both a high reflectance state and a lowreflectance state.
 9. The electrochromic device according to claim 8,wherein the electrochromic medium comprises at least one solvent, ananodic material, and a cathodic material.
 10. The electrochromic deviceaccording to claim 8, wherein the self-cleaning, hydrophilic coating issufficiently hydrophilic such that water droplets on a front surface ofthe self-cleaning, hydrophilic coating exhibit a contact angle of lessthan about 30 degrees.
 11. The electrochromic device according to claim8, wherein the self-cleaning, hydrophilic coating is sufficientlyhydrophilic such that water droplets on a front surface of theself-cleaning, hydrophilic coating exhibit a contact angle of less thanabout 20 degrees.
 12. The electrochromic device according to claim 8,wherein the self-cleaning, hydrophilic coating is sufficientlyhydrophilic such that water droplets on a front surface of theself-cleaning, hydrophilic coating exhibit a contact angle of less thanabout 10 degrees.
 13. The electrochromic device according to claim 12,wherein the self-cleaning, hydrophilic coating includes at least onelayer of photocatalytic material having a thickness of less thanapproximately 300 angstroms.
 14. The electrochromic device according toclaim 12, wherein the self-cleaning, hydrophilic coating includes atleast one layer of photocatalytic material having a thickness of lessthan approximately 500 angstroms.